The major question between the Canon 18-55 mm II and the 17-55 mm USM lenses is one of quality and pricing. Examining them side by side, viewing their quality, and their end results can help to determine if the increase in price for the USM lens is actually worth the money.
Specifications
The major differences are those to be expected between a USM lens and the lower-end variety from Canon. The USM lens has a focusing ring, a different-shaped aperture diaphragm and a Super-UD glass element that is supposed to provide better image quality at all focal lengths. With an additional two lens elements, it is of a better build quality than the II lens, with more features. Both come with Canon's image stabilizer technology and all of the necessities that come bundled with most Canon optics. Weightwise, the II lens comes in at around seven ounces, while the USM lens is over a pound. This is to be expected from the various materials. While the USM lens can drop to f/2.8 the II is stuck at f/3.5--not too much of a difference technically but enough to be noticeable in daily use with the trained eye. The USM lens is also slightly larger, and its minimum focal distance over a foot where the II lens is just below 10 inches.
Uses
With near identical technical settings, these two lenses have very similar uses. The big difference would have to come in the USM lens's ability to focus at a quicker rate making it the better candidate for sports photography or nature work where you can't waste time correcting. The USM lens also has the modified aperture diaphragm which provides some interesting changes in blurred out photos that play around with depth-of-field. An up for the II lens' side, though, is that its price--about $600 less than the USM in 2009--allows it to be a beater lens that you wouldn't mind taking to the beach, leaving attached to your Rebel Ti, and setting down in the sand while you took a swim (almost).
Pros
Some of the things said most often about the USM lens is its durability, consistent output and fast focus--all things that a good USM lens from Canon should be. Its rugged construction really seems to get the vote time and again with dozens of users telling one story or another of how they've dropped it and it amazingly stood up to the beating. Some even claim that the quality is better than some of the L glass they have from Canon.
The biggest pro with the II lens is its price, but that is to be expected. Many like the lens's lightweight construction as well since it keeps their camera easy to tote around. A lot of users appreciate this lens as a good travel companion as well, something you don't mind carting around into situations where you wouldn't want your expensive glass.
Cons
Some users complain of lens creep with the USM lens as well as the weight. The first problem is covered by the warranty. The weight comes with the build quality and can probably be ignored in the bigger picture. A few users also complain about lens flare and blurry focus after extended use, but both seem to be problems that come with inadequate care of your lenses.
Unfortunately for the II lens, the good price tag comes with sacrificing some build quality, and the durability of the lens doesn't seem to hold up to some of the other Canon products. Its cheap casing is easily cracked, and light leaks can form from the slightest bumps. With the cost of replacing the lens being so low this might not be a problem if you can afford to swap whenever a problem arises.
Summary
Build quality is a big issue between these two lenses, but the question really comes down to the features that a user would need. If you are an amateur or enthusiast, then you would definitely want to give the II lens a try, particularly if you are on a budget. However, if quality and durability are what you are looking for and you can afford a higher-priced lens, then you should definitely go for the USM option.
Tags: from Canon, about lens, aperture diaphragm, build quality, come with